In machining the threaded coupling ends of sucker rods which are employed in pumping oil from deep wells, e.g. sometimes extending to depths as great as 14,000 feet; extreme precision is required to assure pumping operation under the high tensile loads involved without any possibility of "working" at the coupling joints which might cause failure and separation within the well. Precision threads axially normal to shoulder engaging surfaces of the rod ends for press fit machining with end surfaces of couplings free of any radial plane angularity which could limit full contacting engagement of adjacent coupling surfaces is a critical requirement.
Conventional prior art systems for machining the ends of sucker rods have employed typical barstock machines with rotating rod and stationary tooling techniques. Rotary indexing heads hold the tooling for successive operations on rod lengths in the range of 23 to 30 feet and rod diameters in the range of 5/8" to 11/8".
Limitations in the speed and accuracy of performing the required successive operations led to the development of an alternative "unwrapped" system wherein each rod is transported horizontally to successive stations at each of which the rod is axially moved to a shoulder gauge stop and clamped against rotation whereupon rotating tooling performs the required operation. While such system has proved commercially successful in improving productivity and accuracy over the prior conventional rotating rod machining method, the requirement for clamping, unclamping and relocating the axial position of the bar at each station has failed to achieve the total benefits of the present system wherein the bar end is clamped and retained in a single accurately located fixturized pallet throughout all operations.
Thus, in clamping and releasing at each station, initial axial registration of the rod takes place against a forged shoulder surface followed by lateral clamping and removal of the axial gauge to provide access for rough machining the shoulder. Due to variations in forgings, clamp release of the rod for transport between stations, possible different rotational orientation at successive stations, possible rebound from the axial gauge surface, and other potentials for mislocation, maximum precision in relative inter-station location of the rods has involved practical tolerances in the order of plus or minus 0.020" even in the absence of any malfunction of the locating tooling. Furthermore, occasionally where workpiece variations cause gross mislocations, substandard machining out of tolerance may result in an undesirable percentage of scrapped sucker rods.